1. Field of the Invention
The present invention relates to double walled pipe structures.
2. Description of Related Art
Such double walled pipe structures are often used for undersea pipelines in the oil extraction industry. Such pipelines lead from the undersea oilfield extraction point to a floating or subsea distribution point. The crude oil carried by these pipes normally emanates from beneath the surface at an elevated temperature. typically 110 to 190.degree. C. If the crude oil is allowed to cool, lower melting point fractions will solidify and prevent further flow along the pipe. Hence, it is important to maintain the elevated temperature of the crude oil, at least until an initial separation stage can be effected.
This is normally achieved by insulating the pipeline. and FIG. 1 shows a known double walled pipe structure comprising an inner flow pipe 10 held concentrically within an outer sleeve pipe 12. Crude oil 14 flows within the flow pipe 10. An insulating material 16 is placed in the annular region between the flow pipe 10 and the sleeve pipe 12. The entire assembly is shown resting on the seabed 18.
The sleeve pipe 12 provides additional mechanical strength, and also prevents water from contacting the insulation material 16. Such insulation materials are commonly porous and would hence lose their insulating ability if they become wet. IT is therefore important to provide an adequate seal against longitudinal Ingress of water along the pipeline, and our earlier international application no. PCT/GB96/01129 filed May 13th, 1996 discloses an annular bulkhead suitable for such purpose.
An inevitable result of maintaining the crude oil at elevated temperature is that at least the flow pipe 10 will be subjected to thermal expansion. This will result inter alia in either an extension of the longitudinal length of the pipeline, or an increase in the longitudinal stress in the pipeline. Such stresses as may result from an increase to 190.degree. C. are capable of causing plastic deformation in a steel pipeline. There are two generally accepted means of absorbing these thermal expansion problems; the first is to bury the pipeline beneath the seabed hence fixing it in position and preventing deformation. The other is to allow the initially straight pipeline shown in FIG. 2a to adopt a meandering course as shown in FIG. 2b and hence take up the increase in length. It is important that the longitudinal extension is maintained within acceptable limits, else a kink such as shown at 20 in FIG. 2c can develop. Such kinks, or areas of excessively high curvature, can cause the pipeline structure to exceed its design limitations
Double walled pipe structures are effective in limiting the overall longitudinal extension, as shown in FIG. 3a. In the absence of longitudinal extension, the flow pipe 10 will be at an elevated tension shown at point 22, whilst the sleeve pipe 12 at ambient temperature will be at zero tension, shown at point 24. If both pipes are allowed to extend longitudinally, the composite structure will adopt a position in which the compressive tension in the flow pipe 10 balances the tensile tension in the sleeve pipe 12, i.e. at point 26. This point is a compromise situation in which the tension T.sub.R in both pipes is lower than a confined unextended pipeline, and the resultant extension is less than in an unconstrained single walled pipeline.
In order for this behaviour to be predictable, it is important that the flow and sleeve pipes 10 and 12 remain in longitudinal register. i.e. that individual elements of the pipe do not become longitudinally separated. The reason for this is shown schematically in FIG. 3d, in which it has been assumed that part of the sleeve pipe 12 is constrained, for example by interaction with the seabed, and that the corresponding part of the flow pipe 10 has become free to move longitudinally relative to that part of the sleeve pipe 12 This means that extension of the sleeve pipe 12 takes place effectively along a shorter length identified as 28, whilst extension of the flow pipe takes place along the entire length. The result of this is that a smaller extension of the sleeve pipe 12 will cause the same increase in tension, and the balance point 26' is at a significantly greater tension in both pipelines. This tension is greater than the original design tension arrived at in FIG. 3a and must therefore be avoided.
The maintenance of both pipelines in longitudinal register is commonly referred to as "longitudinal compliance". It will be clear that longitudinal compliance is achieved by ensuring adequate transfer of longitudinal forces between the two pipes.
FIG. 4 shows a method by which this has previously been achieved. A double walled pipeline is supplied in 24 meter lengths, which is comprises two 12 meter lengths of concentric pipes 10, 12 which are joined at their ends by annular castings which serve to join both pipes and lead to the resultant outlet of diameter corresponding to the flow pipe 10. This can be connected end to end to a similar structure. At the joins, a cylindrical cover 30 is welded in place to give a smooth external profile. Insulation 16 is usually fitted beneath the cover 30. Thus, a direct steel to steel link is formed between the two pipes.
The difficulty inherent in such a structure is that the castings are an integral part of the flow line 10 which is carrying a fluid at an elevated temperature and pressure. The castings must therefore meet the required design standards for the inner pipe, which requires careful control of the cast conditions. This difficulty inevitably increases the cost.
In the system described in our application number PCT/GB96/01129, such castings were omitted and the bulkheads disclosed were used to seal in place hollow alumina-silicate microspheres as the insulating medium 16. Such microspheres can, when compacted, provide a significant shear force transfer between the two pipes 10, 12. Hence, such a compacted insulator was capable of ensuring longitudinal compliance. The system disclosed in PCT/GB96/01129 relied on this effect.
Other solid insulating materials are generally polymeric and unable to withstand the elevated temperatures involved without degrading within the lifetime of the pipe. Fibrous insulation materials such as mineral wool have substantially no shear strength whatsoever and cannot therefore contribute to longitudinal compliance.